Ionic Liquids

Ionic Liquids; Synthesis, Properties and Applications
State of the Art
Submitted
to the Committee of Organic Chemistry
In Partial Fulfillment for Full Professor Post
By
Dr. Maher Ibrahim Nessim
Associate Professor
Analysis and Evaluation Department,
Egyptian Petroleum Research Institute
Cairo – Egypt
2018
Contents
Item Page
Cover 1
Contents 2
Part I. Introduction 3
History 3
Definition and structure of Ionic Liquids 5
Synthesis of ionic liquids 9
Halogenated ionic liquids 9
Halogen free Synthesis 10
Protic Ionic Liquids (PILs) 16
Functionalized Ionic Liquids (FILs) 17
Microwave Synthesis 21
References of part I 22
Part II. Physicochemical properties of Ionic Liquids 28
References of Part II 32
Part III. Applications of Ionic Liquids 35
Electrochemical applications 33
Ionic Liquids in tribology 37
Ionic Liquids in synthesis 39
Applications of Ionic Liquids in Corrosion of steel metal 45
Applications of Ionic Liquid in Carbon Dioxide Capture 47
Conclusion 49
References of Part III 50
Part I
Introduction:
History:
Room temperature ionic liquids (RTILs) have been given careful consideration by academia and industries because of their environment-friendly properties, particularly their abilities in green chemistry. Nonetheless, ILs ought not be claimed as ‘green’ solvents simply because they have negligible vapor pressure (and thus do not emit the harmful and toxic vapors of many volatile organic solvents). Factors such as biodegradation, poisonous quality, and reusing of the IL must also be considered. It is about a long time, (nearly 100 years), since an IL EtNH3+ NO3- which has a melting point of 12 oC, was first reported in 1914, Fig (1) 1.

Fig 1: Structure of ethaminium nitrate
In 1970s, alkylpyridinium or 1,3-dialkylimidazolium haloaluminate salts were prepared. These salts were formed by mixing aluminum halides with the corresponding pyridinium or imidazolium halides 2. Specifically, in 1982 John Wilkes presented tetrachloroaluminate ionic liquids based on 1-alkyl-3-methyl-imidazolium as cation, Figure (2) 3.

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Fig. 2: 1,3-Dialkyl-1H-imidazol-3-iummonoaluminium(III)tetrachloride
Unfortunately, these prepared ILs are very sensitive to atmosphere moisture. In the 1990s, stable compounds with imidazolium-based cations incorporated with tetrafluoroborate or hexafluorophosphate anions were considered as a noteworthy point of reference in the development of ILs. This can be named as the second generation of ionic liquids, (which is yet a standout amongst the most broadly utilized cations today), Figures (3, 4) 4, 5.
Fig.3: 1,3-Dialkyl-1H-imidazol-3-iumhexafluorophosphate Fig.4: 1,3-Dialkyl-1H-imidazol-3-iumtetrafluoroborateate
The third generation, which sometimes featured in literature under the term “designer solvents” or “task-specific ionic liquids”, was presented by Jim Davis and others in 1998 6 figure (5).

Fig. 5: Task-specific ionic liquid 1-(2-((2,4-dichlorobenzyl)oxy)-2-(2,4-dichlorophenyl)ethyl)-3-methyl-1H-imidazol-3-iumhexafluorophosphate
Via careful selection for the different combined cations and anions it was possible to synthesize thousands of ILs. On April 2000, Advanced Workshop on Green Industrial uses of Ionic Liquids was held in Heraklion, Crete, Greece. This was the first international meeting devoted to research in ionic liquids (salts with melting points below 100 oC) 7. It was proposed to explore the promise of ionic liquids as well as to set a research agenda for the field. Since 2000s, more than six thousand papers related to ILs have been published, with more than 1,000 in 2004, 1,300 in 2005, and nearly 1,900 in 2006 8 Figure (6). Until now, the most famous cations are 1,3-dialkylimidazolium-, N,N-dialkylpyrrolidinium-, N,N,N,N-tetraalkylammonium-, and N-alkylpyridinium- based. Concerning their anionic counterparts, tetrafluoroborate (BF4-), hexafluorophosphate (PF6-), bis(trifloromethylsulonyl) amide (NTf2-), and triflate (OTf-) are the most well-known. Figure (7) shows some cations and anions commonly used for the preparation of RTILs 9, 10.

Fig. 6: Annual growth of published papers on Ionic Liquids, by SciFinder

Figure 7: Cations and anions
The significance of ionic liquids is elucidated by many scientific programmes and labs. Some of them, the Japanese “Science of ionic liquids”, the Power, Environmental and Energy Research Center PEER, the Queen’s University Ionic Liquid Laboratories, and the German priority programme “Ionic liquids”. Likewise, there are several conferences are being conducted on this subject. It is moreover illustrated by the fact that many books on the subject exist. (Tpoics in ionic liquids 11).

Definition and structure of Ionic Liquids:
Ionic Liquids are characterized as salts with a melting point below the boiling point of water and They are completely ionic. Most of ionic liquids are liquids at room temperature, so they are called room temperature ionic liquids (RTILs). Significantly, they are compounds consisting of abundant ions that exist in the liquid state at room temperature 12, 13. Ionic liquids consist of bulky and asymmetric organic cations and organic or inorganic anions. Whereas the combinations of the different cations and anions can significantly change the physicochemical properties of the IL generated 14.
2.1. Cations:

2.1.2. Six membered and Benzo-Fused Heterocyclic Cations:
Figure (9) demonstrates a few cations with aromatic character that have been investigated as ionic liquids. Gordon et al. 16 have reported a series of pyridinium hexafluorophosphate salts with long alkyl chains (C12–C18), some of which melt below 100 °C. A more recent area of focus is the viologen family of ionic liquids. While most viologens are very high melting solids, there is a handful that do exhibit much lower melting points, however not exactly room temperature. Benzotriazolium based ILs are frequently good solvents for aromatic species 17.

Fig. 9: Six membered and benzo-fused heterocyclic cations
2.1.3. Ammonium, Phosphonium and Sulfonium Based Cations:
Tetraalkylammonium (Fig. 10) salts have been well known. As far as their utilization as RTILs, earlier studies led to the conclusion that longer alkyl chains were required to get room temperature melting points. Phosphonium RTILs are certainly known and finding developing applications in organic synthesis 18. The hydrogen sulfate salt of the tributyldecylphosphonium cation is a room temperature liquid 19. Phosphonium salts are generally more thermally stable than ammonium salts 20. They are regularly made by alkylation of the parent phosphine 21. One of the slightest studied types of RTILs are those based on the trialkylsulphonium cation.

Fig. 10: Ammonium, phosphonium and sulfonium cations
2.1.4. Functionalized Imidazolium cations:
Ongoing advances in ionic liquids research has provided routes for accomplishing functionalized ionic liquids (Fig. 11) in which a functional group is covalently attached to the cation or anion of the ionic liquid, particularly to the two N atoms of the imidazole ring. It is expected that these functionalized ionic liquids may additionally enlarge the application scope of ionic liquids in chemistry.

Fig. 11: Functionalized imidazolium cations
2.1.5. Chiral Cations:
There are growing numbers of reports demonstrating that chiral ionic liquids may be useful in many areas of science and technology, though synthesis and utilization of chiral ILs is in its beginnings. For example, the use of ephedrinium-based chiral ILs (Fig. 12) as a gas chromatography stationary phase has been reported 22.

Fig. 12: (1S, 2R)-(+)-N,N-dimethylephedrinium ion
2.1.6. Liquid Crystal Cation:
Ionic liquid crystals are a class of liquid-crystalline compounds that comprises of cations and anions. The ionic structure implies that some of the properties of the ionic liquid crystals differ significantly from that of conventional liquid crystals. Typical for ionic liquid crystals is the ion conductivity. The ionic interactions tend to stabilize lamellar mesophases. Ionic liquid crystals systems can be considered as materials that combined a relatively low melting point with a large mesophase range i.e. the properties of liquid crystals and ionic liquids 23. Worldwide serious intense research activity in the field of ionic liquids is presently ongoing 24-31. The n-alkylammonium cations are the simplest type of liquid-crystalline ammonium cation (Figure 13) 13-35. Imidazolium ionic liquid crystals are also reported (Figure 14) 36.

Fig. 16: sulfate, sulfonate, Phosphate, phosphonate and phosphinate anions
3. Synthesis of Ionic Liquids:
3.1. Halogenated ionic liquids:
Most of halogenated ionic liquids are commonly prepared by quaternization of alkylamines, imidazoles, or phosphines, often employing alkyl halides as alkylating agents.
3.1.1. Alkyltrimethylammonium bromide:
As an example of the of the most conventional ionic liquids is the Alkyltrimethyl-ammonium halide which is produced via quaternization of trimethyl amine with alkyl halides (with different chain length) Figure (17).

Fig. 18: Synthesis of Alkyltrimethylphosphonium halide ionic liquid
3.1.3. 1-Alkyl-3-methylimidazolium halides:
This type of ionic liquid is simply prepared via reaction of 1-methylimidazole with different alkyl halides, under a reflux for 12 hrs., in presence of acetonitrile as solvent. Figure (19) 54.

Fig. 19: Synthesis of 1-Alkyl-3-methylimidazolium halides ionic liquid
3.2. Halogen Free Synthesis:
The majority of these ILs are usually produced by simple N-alkylation of N-alkylimidazole, often utilizing alkyl halides as alkylating agents, followed by association with metal-halides anion metathesis (Scheme 1) 55. In this method, produced ionic liquids of high purity materials is somewhat problematic because of contamination by residual halide. The presence of halides in the resulting ionic liquids can definitely change the physical properties 56 and may bring about in catalyst poisoning and deactivation 57. Accordingly, various synthetic methodologies have been devised to synthesize halide free ionic liquids.
Scheme1 Scheme 2
Dupont et al. 58 reported the direct synthesis of 1,3-disubstituted imidazolium tetrafluoroborate ionic liquids utilizing one step (Scheme2). Reaction of formaldehyde with n-butylamine followed by adding methylamine, aqueous HBF4 solution and aqueous glyoxal solution affords, in 66% yield, a mixture of C4mimBF4, BBIMBF4 and C1mim BF4 in a molar ratio of (5:4:1). Another method to produce free halogen ionic liquids was reported by Deyab et al. 59 (Scheme 3).

Scheme 3
Three other direct syntheses of halide free ionic liquids are categorized as follows:
3.2.1. Synthesis of Ionic Liquids via N-Heterocyclic Carbene (NHC) Intermediates:
Carbenes are molecules which have a lone pair of electrons on a carbon atom; this thusly renders them highly reactive. Thus, carbenes are useful intermediates in the synthesis of chemical compounds. The synthesis of ionic liquids via carbenes can be accomplished either by the reaction of NHC adducts with acids (Scheme 4a), or reaction of NHC-organometallic intermediates with acids (Scheme 4b).

Scheme 4: Synthesis of ionic liquids via a carbene
3.2.1.1 Preparation of NHCs and Reaction with Acids:
Various methods to produce imidazole carbenes have been reported. Recently, Seddon and Earle reported a simple methodology to produce the imidazolium carbene 2 from an imidazolium chloride 1 (Scheme 5) 60. These carbenes can be utilized to synthesize the corresponding imidazolium salts by a simple reaction with the protonated forms 3 – 8 61 of the required anion (Scheme 5). The advantage of producing imidazolium salts by this process is that it generates ionic liquids which are not contaminated by unwanted halide ions or metal ions.

Scheme 5
The 1,3-dialkylimidazolium-2-carboxylates (8) readily react with dry methanol, benzoylacetate and benzaldehyde in the presence of equal amounts of NaBF4, KPF6 or NaBPh4 affording the corresponding 1,3-dialkylamidazolium salt according to Scheme (6) 62.

Scheme 6
Recently, Rogers et al. 63 reported the synthesis of intermediate 1,3-dimethylimidazolium-2-carboxylate (8a), (scheme 7) via the reaction of dimethyl carbonate (DMC) with 1-methylimidazole (9), in which the acidic C2-hydrogen of the resulting 1,3-dimethylimidazolium cation is abstracted by the methyl carbonate anion, leading to the heterocarbene and HOC(O)OMe which is unstable and gives rise to MeOH and CO2. Nucleophilic attack on CO2 by the carbene is the only favored process and leads to the formation of (8a) which can be reacted with any of the acidic components in Scheme 5.

Scheme 9: Halogen free synthesis of phosphonium ionic liquids
Ammonium dialkylphosphates were first described in 1952 65. The alkylation products of pyridine and trialkylphosphates were described to be salts with very low melting points in 1989 66. Recently, Cytec documented patents on the synthesis of imidazolium based dialkylphosphate ionic liquids 20. They synthesized tetrabutylphosphonium dibutylphosphate, N,N-dimethyl-imidazolium dimethyl-phosphate, N-methyl-N-butylimidazolium dibutylphosphate, and N-methyl-N-ethyl-imidazolium ethylethanephosphonate. Downard et al 20 reported synthesis of phosphonium, phosphates, phosphonates and phosphinates by direct reaction of phosphines or imidazoles with alkylating agents such as dialkylsulfate, trialkylphosphates and dialkylphospho-nates and monoalkylphosphinates (Scheme 8).

Scheme 10: Direct alkylation via sulfonates, phosphonates and phosphinates
Recently, to build up an optimized synthetic protocol for the alkylation of 1-alkylimidazole compounds with trimethylphosphate, the kinetics of the synthesis of 1-dimethylimidazolium dimethylphosphate were studied in detail 65. In 1991, Whitesides synthesized tris(2-carboxyethyl) phosphine hydrochloride 67. Since then, air sensitive phosphonium salts have also been synthesized by the reaction of phosphine with a solution of aqueous HBF4 (Scheme 11).

Scheme 11
3.2.3. Sulfur Based Ionic Liquids:
Direct synthesis of sulfur based ionic liquids is considered as a useful synthetic method for developing halogen free ionic liquids. The synthesis of sulfur based ionic liquids can generally be split in to two types: (1) sulfonate and (2) sulfate based ionic liquids. Note that, the sulfate based salts are more common than the sulfonates.

3.2.3.1 Sulfonate Based Ionic Liquids
Direct quaternization of an alkylsulfonate into imidazole is useful to different anions via metathesis in the halogen free method. The alkylation of N-alkylimidazoles with alkylsulfonate can be performed under solventless conditions at room temperature, affording corresponding 1,3-dialky-limidazolium alkanesulfonate salts as crystalline solids 55. The alkane sulfonate anions can easily be substituted by a series of other anions by metathesis (Scheme 12).

Scheme 12
Jonathan and Mikami 68 detailed a straightforward synthesis of new chiral ionic liquids bearing an imidazolium core, an easy and efficient method (6). Commercially available ethyl lactate was converted into its triflate derivative which upon reaction with 1-methylimidazole gave the triflate salt as a solid in excellent yield. At this point anion metathesis was performed to obtain different anions with imidazole cation (Scheme 13).

Scheme 13
3.2.3.2 Sulfate Based Ionic Liquids:
The hydrophilic C2mimEtSO4 has received considerable attention, becoming one of the first commercially ionic liquids available in bulk. Ionic liquids containing methyl- and ethyl- sulfate anions can be easily and efficiently produced under ambient conditions by the reaction of 1-alkylimidazoles with dimethyl sulfate and diethyl-sulfate (Scheme 14) 44.

Scheme 14
Quite recently, trans-esterification of 1-alkylimidazoliumsulphate was carried with functionalized and non-functionalized alcohols, to the corresponding new alkyl-sulfates (scheme 15) 69.

Figure 20: Protic imidazolium and alkoxyimidazolium based ionic liquids
3.4. Functionalized Ionic liquids (FILs):
During the last eight years 73, 74, varieties of functionalized ionic liquids (Fig. 21) expressly categorized as being “task-specific” ionic liquids have been designed and synthesized for specific purposes which will be studied later. The salts are characterized as functionalized ionic liquids when they are ionic liquids in which a functional group is covalently attached to (1) the cation, (2) the anion (few), or (3) a zwitterionic form of the salt. It is ordinarily the cation that bears the reactive part.

Figure 21: Types of functionalized ionic liquids
Until recently, the method used to incorporate the functionality into the ionic liquid was always displacement of the halide from an organic halide containing the functional group by a parent imidazole, phosphine, etc. (Scheme 17).

Scheme 17
Davis and Rogers 75, 76 have reported in their works the synthesis of imidazolium salts with urea, thiourea and thioether groups in one of the N-alkyl substituents (Fig. 22).

Figure 22: Imidazolium salts with urea, thiourea and thioether groups
Recently, novel sulfonyl-functionalized ionic liquids with SO3H and SO2Cl groups have been synthesized. First reported were Bronsted-acidic functionalized ionic liquids bearing an alkane sulfonic acid group in an imidazole or triphenylphosphine cation (Fig. 23a) 77 or in a pyridinium cation (Fig. 23b) 78.

Scheme 18
Recently, neutralization between C2mimOH and amino acids produced ionic liquids with amino acids as anions 79. The reaction of tetrabutylphosphonium hydroxide P(C4)4OH with amino acids yielded tetrabutylphosphonium amino acids P(C4)4AA, including glycine, l-alanine, l-b-alanine, l-serine and l-lysine 80. The esters or amide derivatives of bromoacetic acid were commercially available, formed in one step via the reaction of bromoacetyl bromide with alcohol or amine 81–83. Ester and amide side chains can be produced easily via this route. On the other hand, functionalized ionic liquids with electrophilic alkene-type appendages were synthesized.

Scheme 20: Functionalized ionic liquids with NH2 group
3.4.2. Functional Group Covalently Tethered to the Anion:
Functionalized ionic liquid as electroactive compounds, tetraalkylphosphonium polyoxo-metalates, were synthesized by the substitution of sodium tungstate with tetraalkylphosphonium-bromide (C6H13)3P(C14H29)2W6O19 89.

3.4.3. Zwitterionic-Type:
Ohno et al. 90 reported that the synthesis of zwitterionic-type molten salts and their polymers (Scheme 21). Since zwitterionic-type salts have both cation and anion in intramolecular form, these ions cannot migrate with the potential gradient.

Scheme 21: Zwitterionic-type molten salts and their polymers
3.5. Microwave synthesis:
The preparation of 1,3-dialkylimidazolium halides via the classical heating method in refluxing solvents requires several hours to afford reasonable yields and uses a large excess of alkylhalides/organic solvents as the reaction medium. The shortened reaction time, cleaner work-up procedure and unique transformations achieved by microwave synthesis of ionic liquids can be impressive. Microwave synthesis can be classified into the following steps:
3.5.1. Quaternization step:
The first report on the microwave-assisted synthesis of imidazolium-based ionic liquids appeared in 2000 91. Later Varma et al. 92, 93 reported a microwave assisted preparation of series of ambient temperature Cnmim-type ionic liquids by the reaction of 1-methylimidazole with alkyl halides/terminal dihalides under solvent-free conditions using a microwave oven as an irradiating source. The instrument used was a common household microwave oven equipped with inverter technology. The synthesis of ionic liquids has also been carried out by irradiating equimolar amounts of N-methyl imidazole and an alkylating agent in open containers, (Fig 26).

Part II
Physicochemical properties of Ionic Liquids:
A significant amount of research on ionic liquids has been performed, from fundamental studies to application, due to their unique characteristics, such as non-volatility, excellent catalytic performance, and good solubility for many materials. These unique physical and chemical properties depend on the special microstructure and the complex interactions within the ionic liquid systems. However, there are thought to be at least a million types of possible ionic liquid 1. Therefore, it is very challenging to screen and design functional ionic liquids for specific applications. Anyone developing ionic liquids for technological applications must face this challenge. Efforts now being made to model and predict the properties of ionic liquids 2.

So, currently, research on the physicochemical properties of ionic liquids has been closely combined with development of their applications. However, a large amount of data is dispersed in various journals, reports, books, patents, and so on. The ionic liquid databases provide access to the scientific data needed to refine universal property relationships, reveal the inner relationship between microscopic structure and macroscopic properties, design new ionic liquids effectively, and develop applications for ionic liquids. In the following sections, we will discuss the key relationships of the physicochemical properties of ionic liquids, especially melting point, density, viscosity, and surface tension, including an assessment of the current situation and identifying trends for ionic liquids in the future.

The main properties of ILs for their applications are the following:
Extremely low vapor pressure:
It is usually assumed that the vapor pressure of the ionic liquids is zero at room temperature. However, almost no vapor pressure data for ionic liquids have been reported so far 3. So, most of ionic liquids are considered safer and more environmental than organic solvents. Some studies have been carried out showing that ionic liquids can in fact be vaporized at low pressures and high temperatures 5, 6. For some ionic liquids, vapor pressure data could be obtained at temperatures below 300 °C 4, 7-8. A rapid screening method for vapor pressure measurement is the use of a thermogravimetric analyzer (TGA). This method has the advantage that only a small amount of substance is needed. The TGA method has been used to investigate the vapor pressure of solids as well as liquids 9.

Thermal Stability:

One of the most important properties of an IL is the temperature range over which it remains a liquid, that is, between its solidification at low temperatures and decomposition at high temperatures. Being advantageous over conventional organic solvents, ILs have attractive physicochemical properties such as excellent thermal and chemical stability 10. The thermal stability of ILs is limited by the strength of their heteroatom-carbon and their heteroatom-hydrogen bonds, respectively. The first serious discussions on the thermal stability of ILs emerged from investigations using thermogravimetric analysis (TGA) 11. In most cases, decomposition occurs with complete mass loss and volatilization of the component fragments.

Polarity and Solubility:

Many ionic ILs possess the ability to dissolve a wide range of inorganic and organic compounds 12. The polarity of many ILs is intermediate between water and chlorinated organic solvents and varies, depending on the nature of the ionic liquid components. Because of the diversity of their component organic ions, it is possible to tune their physico-chemical properties, including polarity, viscosity and melting point. This promising diversity suggests that appropriate ILs would be polar solvents for cellulose. ILs such as N,N9-dialkylimidazolium chloride salts (RR9imCl) dissolve cellulose 13 and other biomacromolecules 14. The solubility of ILs in water depends on the nature of the anion, temperature and the length of the alkyl chain on the organic cation. Tetrafluoroborates, chlorides, nitrates, and trifluoroacetates display complete miscibility with water, whereas hexafluorophosphates, triflimides, and other perfluorinated anions impart very low solubilities in water. The hydrophilic/ hydrophobic behavior is important for the solvation properties of ILs as it is necessary to dissolve reactants, but it is also relevant for the recovery of products by solvent extraction. Empirical solvent polarity scales give insight into solvent-solute interactions.

Electrochemical Stability.

ILs often have wide electrochemical potential windows, they have reasonably good electrical conductivity and high ionic conductivity up to 0.1 S cm-1 10. The electrochemical window of an ionic liquid is influenced by the stability of the cation against electrochemical reduction-processes and the stability of the anion against oxidation-processes. Their electrical stability with large electrochemical windows typically over 4V wide, 15 making them especially suitable for diverse electrochemical applications such as electroplating of base metals, rechargeable batteries, dye-sensitized solar cells (DSSCs) and other electrochemical devices 16.

Non-flammability
ILs are safe for hanging, have high mobility, high heat capacity and cohesive energy density, low toxicity and non-flammability 17. So, they can be used to develop a truly safe large size lithium ion cell suitable for electric or hybrid vehicles 18.

Miscibility
Many ILs are immiscible with either water or organic solvents, so their use in creating biphasic systems has attracted interest, especially for separation purposes. Therefore, novel liquid-liquid partitioning systems (water-ionic liquids) have been considered 19, 20.

Catalytic properties.
The catalytic properties in organic and inorganic synthesis have been widely described 21. The increased interest in ionic liquids by chemists and technologists clearly is due to the utility of ionic liquids as solvents for reaction chemistry, including catalytic reactions 22. Until now, ILs have shown promise in selective catalytic processes for organic and organometallic chemical reactions 23.

Surface Active properties:
One of the interesting properties of ILs is their inherent amphiphilicity which places them in the class of ionic surfactants. In this regard, the aggregation behavior of amphiphilic ILs in aqueous has been investigated by many researchers 24–27. Ionic liquids of the type 1-alkyl-3-methylimidazolium halides RmimX, with a long chain (R) are expected to be surface active ionic liquids, (SAILs) 24. Indeed, conductimetric, potentiometric, surface tension and volumetric (from solution density) studies have indicated that C10mimBr aggregates in water and, at low concentrations, behaves as a classic cationic surfactant (Fig. 1) 28, 29; surface tension measurements have indicated a similar behavior of C16mimCl and C16mimBF4 both in aqueous solution, and in the IL ethylammonium nitrate 30.

C10MeimBr C16MeimX
Figure 1: Structure of SAILs, C10MeimBr and C16MeimX
The mixed surfactant systems comprising cationic and anionic surfactants are known to exhibit synergism and show a rich polymorphic behavior at room temperature in the form of globular micelles, spontaneous vesicles, and solid precipitates etc. 31, 32. When oppositely charged surfactants are mixed together, the strong electrostatic interaction between the head groups results into reduction in area per head group leading to formation of diverse bilayer assemblies depending on the mixing ratios, composition and added electrolytes 33.

On the other hand, there exists a very few reports in literature pertaining to adsorption and micellization behavior of surfactant like ILs with other ILs or conventional surfactants 34–36. Zheng et al. described the phase behavior and vesicle formation in catanionic system containing long chain IL1-dodecyl-3-methylimidazolium bromide C12mimBr and SDS using electrical conductivity measurements and prepared hollow silica spheres using IL-SDS vesicles 37.

Part III
Applications of Ionic Liquids (ILs):
The third part aims to highlight the diverse range of ionic liquid applications with the focus on the properties of ILs that render them suitable for each and a brief discussion of the current state of the art about IL technology. Because of the sheer number of areas where ILs can be applied, the highlighted applications do not aim to be exhaustive nor can the discussion of each area be fully comprehensive; however, I hope this part illustrates the utility of this unique class of compounds.

Electrochemical applications:
As ILs consist exclusively of ions, they are obvious candidates as electrolytes for a range of electrochemical applications. ILs offer many advantages over electrolytes that feature salts dissolved in molecular solvents. First, their low vapor pressures reduce their flammability, making them less of a fire hazard than electrolytes based on organic solvents 1, 2. Their low vapor pressures also mean that they do not evaporate in open systems 3. Second, as they are composed solely of ions, ILs possess much greater concentrations of potential charge carriers relative to dilute salt solutions. Although this could be expected to lead to exceptionally high conductivities, this generally does not occur due to factors such as their substantial viscosity as well as the extent of ion aggregation and correlated ion motion 4, 5. Some ILs, such as the 1-alkyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (CxC1pyrrNTf2) class, possess very large electrochemical windows more than 5.5 V, which increases their compatibility with a wide variety of reagents and electrochemical processes 4, 6-7. Finally, a few ILs possess large liquidus ranges, which enables their application over a wider range of temperatures than many conventional electrolytes 8. These favorable properties have led to ILs being investigated as electrolytes for applications including supercapacitors and batteries.

Consequently, the optimal IL for use as an electrolyte will vary depending on the temperature required for the application. In terms of anion selection, wide electrochemical windows and lower viscosities are generally observed for ILs with fluorinated anions such as NTf2? or tris(perfluoroalkyl)trifluorophosphate (FAP?) 9, 10. About cations, it has generally been found that electrochemical stability increases in the order imidazolium < ammonium < pyrrolidinium < phosphonium 11. Despite their low stability imidazolium ILs have often been found to possess lower viscosities hence higher conductivities than ILs based on other cations. Imidazolium ionic liquids are also generally liquids over a wider temperature range than those based on most other cations. For supercapacitors based on graphene nanosheets, the ionic liquid C4C1pyrrN(CN)2 (Fig. 1) resulted in greater energy and power densities than NTf2?. This is due to the high conductivity and capacitance of the ionic liquid based on the small and less viscous N(CN)2? anion, which compensates for the decrease in size of the electrochemical window.

Figure 1: Structure of 1-butyl-1-methylpyrrolidin-1-ium cation, with different anions
One of the leading examples of the use of ionic liquid electrolytes in supercapacitors is the use of an equimolar eutectic mixture of the ionic liquids N-methyl-N-propylpiperidinium bis(fluorosulfonyl)imide C3C1pipFSI and N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide (C4C1pyrrFSI) 12 (Fig 2).

Figure 2: Structure of (C4C1pyrrFSI, and C3C1pipFSI
One of the main attractions of the use of ILs is their reduced fire and explosion risk, particularly for high-energy batteries. Correspondingly, battery systems are more complex as the electrolyte needs to also be compatible with the redox couple in terms of the potential required and the reactive species present 13. The former requirement can be assisted by the formation of a solid electrolyte interphase (SEI), a surface passivating layer on the electrode composed of insoluble decomposition products arising from the solvent and electrolyte, which can enable the long-term stability of the electrolyte even under thermodynamically unfavorable conditions. An advantage of ILs is that they simplify SEI formation as the cation does not interact with the anode. This means that the SEI is formed exclusively from the decomposition of the IL anion rather than the organic solvent and the electrolyte anion 14. ILs based on the FSI? and NTf2? anions after cycling have been found to produce effective SEIs for lithium electrodes. These courses, the suppression of dendritic lithium formation, leading to short circuits 15.
Ionic Liquids in tribology:
Tribology is the science and engineering of interacting surfaces in relative motion. It includes the study and application of the principles of friction, lubrication, and wear. ILs have become an area of interest for novel tribology studies because of their unique properties, negligible volatility, nonflammability, high thermal stability, and good intrinsic performance. They have been investigated as lubricants and additives. Their ability to reduce friction and wear significantly is of most importance for this application. Their viscosity, thermal stability, and wettability were characterized to establish their potential as a lubricant or additive 16. The formation of effective lubrication films depends on the alkyl chain of the cation and the anion composition for the neat ILs as lubricants and on miscibility with the base oil and atmospheric moisture when used as an additive.

Lubricants:
A lubricant is a substance introduced to reduce friction between surfaces in mutual contact, which ultimately reduces the heat generated when the surfaces move. It may also have the function of transmitting forces, transporting foreign particles, or heating or cooling the surfaces. There are many studies about the performance of different ILs in tribology as lubricants; the ILs used in this field are composed of imidazolium, ammonium, pyridinium, and phosphonium cations, and Cl?, Br?, BF4?, and PF6? anions, with different alkyl substituents on the cations. The anion seems to have more of an effect on tribology properties than the cation 17. The most common ILs are composed of imidazolium cations with BF4? and PF6? as the anion. Br? related ILs were discarded because of their hydrophilicity, which can increase tribocorrosion on the joints 18.
In 2001, Ye et al. 19 studied the performance of C6C1imBF4 and C6C2imBF4 (Fig. 3) as lubricants for steel, aluminum, copper, single crystal SiO2, single crystal Si(100), and sialon (Si–Al–O–N) ceramics; these ILs showed good lubricant behavior compared with standard lubricants.

Figure 3: Structure of C6C1imBF4 and C6C2imBF4
Weng et al. 20 studied several asymmetric tetraalkylphosphonium ILs’ performance on steel/steel contact and compared their performance with high-temperature oils and imidazolium ILs. All the synthesized phosphonium ILs showed similar or better tribology performance but lower thermal stability in contact with the air 20. Jimenez et al. 21 studied the performance of C8C1imBF4 and C6C1imPF6 on titanium/steel contact; they observed that C6C1imPF6 showed good tribology performance at high temperatures and when they exchange steel with ruby, tribocorrosion was avoided (fig. 4).

Figure 4: Structure of C6C1imPF6 and C8C1imBF4
The main drawback of using ILs as lubricants is their reactivity and potential decomposition at high temperatures, which can cause tribocorrosion. In accordance with the good performance shown by the ILs at reducing friction and wear, their use as additives for conventional oils was studied to avoid tribocorrosion. Kamimura et al. 17 studied the performance of additives on ILs (imidazolium, ammonium, and pyridinium). They found that the additives (tricresylphosphate and dibenzyldisulfide) helped to prevent the tribochemical decomposition of the ILs 17. In general, ILs showed lower friction coefficients than conventional oils in the following order: imidazolium > ammonium > phosphonium, but similar wear coefficients as base oils.

Additives:
Many additives are used to impart performance characteristics to lubricants. The additives can act as detergents, defoamers, antioxidants, and antiwear agents. With the use of ILs, the quantity of additives used can be reduced. The formation of effective lubrication films depends on the alkyl chain of the cation and the anion composition and on miscibility with the base oil and atmospheric moisture when it is used as an additive. The performance of ILs as additives is not the same as observed when ILs are used as lubricants by themselves. Friction and wear were reduced on metallic and ceramic surfaces where a few percent of IL were used as an additive for base oils and water. While neat ILs tend to react with metallic surfaces, leading to tribocorrosion, when ILs are used as additives, the tribocorrosion decreased substantially, indicating that the ILs do not cause damage 17, 21.

Ionic Liquids in Synthesis:
To many chemists it may seem daunting to perform reactions in ionic liquids, and the range of ionic liquids or potential ionic liquids available is very large. However, many scientists have found that performing reactions in ionic liquids is straightforward and practical when compared with similar reactions in conventional organic solvents. This is particularly the case when considering reactions normally carried out in harmful and difficult to remove solvents such as dipolar aprotic solvents like dimethyl sulfoxide. With the growing interest in ionic liquids, reactions were initially performed in various chloroaluminate(III) ionic liquids. Their strong solvating ability was an advantage, but their sensitivity to moisture and strong interactions with certain commonly occurring functional groups limited the scope of reactions in these media.
With the discovery of water-stable “neutral” ionic liquids, the range and scope of reactions that can be performed has grown to include most classes of reactions covered in organic chemistry textbooks and most of reactions in ionic liquids are now carried out in these water-stable variants. It is evident that ionic liquids may affect reactivity much more than molecular solvents. They are not only mixtures of two species interacting with each other (anion and cation), each able to give specific interactions with the dissolved reagents and/or activated complexes, but the anion–cation interactions make them complex three-dimensional structures able to exercise unusual effects
3.1. Solvents for Organic Synthesis:
The unusual structure of ILs compared to conventional neutral organic solvents could lead to changes in reaction outcomes for all organic chemical processes. 23.Stoichiometric or, more simply, non-catalytic – reactions are an important and rapidly expanding area of research in ionic liquids. This section deals with reactions that consume the ionic liquid (or molten salt) or use the ionic liquid as a solvent.

Molten salts as reagents:
A number of examples of the use of molten pyridinium chloride (m p 144 °C) in chemical synthesis are known, dating back to the 1940’s. Pyridinium chloride can act both as an acid and as a nucleophilic source of chloride. These properties are exploited in the dealkylation reactions of aromatic ethers 24. An example involving the reaction of 2-methoxynaphthalene is given in Scheme 1 25, 26, and a mechanistic explanation in Scheme 2 26. Pyridinium chloride (PyHCl) has also been used in several cyclization reactions of aryl ethers (Scheme 3) 24, 26. Presumably the reaction initially proceeds by dealkylation of the methyl ether groups to produce the corresponding phenol. The mechanism of the cyclization is not well understood, but Pagni and Smith have suggested that it proceeds by nucleophilic attack of an Ar-OH or Ar-O– group on the second aromatic ring (in a protonated form) 23.

Scheme 2: A mechanism for dealkylation of aryl ethers with pyridinium chloride

Scheme 3: Aryl demethylation reactions followed by cyclization
3.1.2. Diels Alder Reaction:
Important information about ionic liquid properties has also been obtained by the study of reactions occurring through isopolar transition states, such as Diels–Alder reactions 27. Diels–Alder reactions can indeed be efficiently performed using water as reaction medium 28. Recently, ionic liquids have also been used with success as solvents for Diels–Alder reactions. The reactions in ionic liquids are indeed marginally faster than in water but are considerably faster than in diethyl ether. Furthermore, it has been shown that, as with molecular solvents, the presence of a Lewis acid greatly accelerates the reaction and improves selectivity. The acidity of chloroaluminates 29, or ionic liquids containing ZnCl2 and SnCl2 30, have been used to this purpose.
The molecular origin of how ionic liquids influence this reaction is, however, always a matter of controversy. A solvophobic effect, able to generate an “internal pressure” and to promote the association in a cavity of the solvent, has been initially invoked to explain the kinetic and stereochemical behavior of Diels–Alder reactions carried out in ionic liquids 31, 32. A more recent study on the reaction of cyclopentadiene with methyl acrylate in several ionic liquids has, however, provided evidence that the ionic liquid’s hydrogen-bond donor ability increases reactivity and selectivity 33. Moreover, the determination of the selectivity in five C4C1im+-ionic liquids has shown that the nature of the anion also affects the endo/exo ratio. Higher selectivities characterize ionic liquids having the smaller hydrogen-bonding interaction between cation and anion. The endo selectivity has therefore been explained considering that the ability of the cation to hydrogen bond methyl acrylate is determined by two competing equilibria (Fig. 6).

Figure 6: Ability of the 1,3-dialkylimidazolium cation to hydrogen bond methyl acrylate during its reaction with cyclopentadiene.

It is finally worth noting that phosphonium tosylates 34, and more recently pyridinium-based ionic liquids 35, have also been used as solvents for the Diels–Alder reactions of isoprene with methyl acrylate, acrylic acids, but-3-en-2-one and acrylonitrile (Scheme 4).

In phosphonium salts the reactions of isoprene with oxygen-containing dienophiles proceed with high regioselectivity (>99:1), whereas in pyridinium based ionic liquids selectivity and reactivity depend on the ionic liquid anion (BF4? < CF3COO?), on dienophile nature and on the reaction time. The high regioselectivity characterizing many of these reactions has been attributed in both cases to the ability of the ionic liquid to coordinate substituents on the dienophile.

3.1.3. Friedel-Crafts Reaction:
Friedel–Crafts reactions in the ionic liquid system 1-methyl-3-ethylimidazolium chloride aluminium(III) (emimCl–AlCl3) chloride can be performed with excellent yields and selectivities. 36. Such ionic liquid has been shown to demonstrate catalytic activity in reactions such as Friedel–Crafts acylations 37, 38 (Scheme 5), alkylation reactions 39, isomerization of alkanes 40 and the alkylation of isobutane with butene 41. Here, performance for series of reactions of AcCl with carbocyclic aromatic compounds in acidic compositions of emimCl–AlCl3 (Fig. 1) were compared to those with similar reactions in ‘conventional’ molecular solvents.

Figure 7: structure of emim+ cation Scheme 5: Mediated Friedel-Crafts acylation of ferrocene
3.1.4. Esterification:
Ionic liquids are useful media for esterification reactions because they enable vacuum (to remove condensate) and solvent to be used together, creating an opportunity to drive the equilibrium. Also, the properties of ILs can be tuned to be both polar and hydrophobic 42. This refers to the ability of an exciting possibility to solve the polar solutes in a dehydrating solvent. Consequently, ionic liquids are commonly referred to as designer solvents because anions and cations can be changed independently of one another, allowing solvent-solute interactions to be tuned to purpose. Recent work by Tanabe et al. 43 and Sakakura et al. 44 has demonstrated the use of biarylammonium salts as mild catalysts for the esterification of acids with alcohols. High conversions were achieved without the removal of water, demonstrating the potential of salts, in conjunction with hydrophobic solvents, to catalyze esterification and drive conversion. Esterification has already been demonstrated in various ionic liquids. Davis et al. were the first to report the use of IL tethered sulfonic acids to promote Fisher esterification 45, 46 (Scheme 6). This concept of a Bronsted IL being used as an esterification catalyst has subsequently been expanded upon, most recently by Li et al. 47.

Scheme 6: Brønsted acid TSIL
3.1.5. Heck Reaction (Mizoroki-Heck):
The Heck reaction 48 (Fig. 8 is the chemical reaction of an unsaturated halide with an alkene in the presence of a base and a palladium catalyst to form a substituted alkene 49, 50. This reaction was the first example of a carbon-carbon bond-forming reaction that followed a Pd(0)/Pd(II) catalytic cycle, the same catalytic cycle that is seen in other Pd(0)-catalyzed cross-coupling reactions. The Heck reaction is of great importance, as it allows one to do substitution reactions on planar sp2-hybridized carbon atoms.

Figure 8: Heck Reaction
The original reaction by Tsutomu Mizoroki (1971) describes the coupling between iodobenzene and styrene to form stilbene in methanol at 120 °C (autoclave) with potassium acetate base and palladium chloride catalysis (Scheme 6).

Scheme 7: Stilbene synthesis by Heck
Shenghai et. al. 51 reported that, Brønsted acid-base ionic liquids (GILs) based on guanidine and acetic acid are efficient reaction media for palladium-catalyzed Heck reactions (Scheme 8). They offer the advantages of high activity and reusability. GIL plays multiple roles in the reaction: it could act as solvent, as a strong base to facilitate ?-hydride elimination, and as a ligand to stabilize activated Pd species. It was noticed that, the catalyst system based on guanidine ionic liquids displayed excellent activity.

Scheme 8: Synthesis of stilbene derivatives in presence of Guanidine Ionic Liquid
The role of ionic liquids in the Heck reaction “Ionic Liquid Effect” is illustrated as follows:
Performing the Heck reaction in ionic liquids may lead to an increasing in the reaction rate in comparison with “classical” solvents, to a stabilization of the catalytically active species and, in favorable cases, also to a control on the regio- and stereoselectivity of the coupling products. The nature of the ionic liquid however affects not only the kinetic constants of the single steps of this reaction but, probably, determines also the nature of the catalysts or pre-catalysts.

Applications of Ionic Liquids in Corrosion of steel metal:
The use of corrosion inhibitors comprises one of the most economical ways to mitigate corrosion rate and protect metallic materials against corrosion to preserve industrial facilities 52 especially in acidic media 53. The treatment of mild steel corrosion in acidic environment through organic compounds has resulted in considerable savings for the oil industry. Several families of organic compounds, fatty amides 54, 55 pyridines 56-58 imidazolines 59-61 and 1,3-azoles 62-64 have shown excellent performance as corrosion inhibitors. On the other hand, some cationic gemini surfactants were synthesized and applied as corrosion inhibitors for carbon steel 65-67. However, many of these compounds are toxic, and they do not completely fulfill the requirements imposed by the environmental protection standards. This is the reason why in the past few years great efforts have been made by researchers in this area to develop new environmentally friendly corrosion inhibitors 68. Because of the interesting physical and chemical properties of ionic liquids, they have attracted the attention in the past decades. There are several studies that involved ionic liquids as corrosion inhibitors for acid environments which have shown good properties as corrosion inhibitors for mild steel in aqueous HCl. Diego et. al. synthesized five imidazolium-type ionic liquids, containing both N1 unsaturated and N3 long alkyl saturated chains as cations together with bromide as anion, and evaluated them as corrosion inhibitors for acid environment (Fig. 9) 69.

Figure 9: Vinylimidazolium ionic liquids
Kowsari and his coworkers used a task-specific ionic liquid (TSIL) with the molecular structure of a Gemini cationic surfactant to inhibit the corrosion of low carbon steel in 1 M HCl 70 (Fig.10).

Figure 10: The chemical structure of the TSIL
Deyab et. al. modified the chemical structure of 1-decyl-3-methylimidazolium tetrafluoro-borate by increase the alky chain length fromC10 to C12 and the hydrogen in carbon 2 of the imidazole ring, (between the two nitrogen atoms), was replaced by methyl group. Finally, they obtained three new ionic liquids and evaluated the corrosion inhibitions performance of these new compounds 71 (Scheme 9).

Scheme 9: Synthetic procedure of ILs, where n = 10, 12
In 2017, Deyab and his coworkers prepared series of gemini ionic liquids with flexible spacer 72 (Scheme 10) and thence in 2018 they synthesized different series of gemini ionic liquids with rigid spacer 73 (Scheme 11). The reported results improved that gemini ionic liquids with rigid spacer showed greater efficiency than those with flexible spacers.

Scheme 11: Gemini Ionic Liquids with rigid spacer, where R = C12
Applications of Ionic Liquid in Carbon Dioxide Capture:
The main problem with carbon dioxide is its greenhouse effect on the atmosphere, which leads to increasing temperatures. Carbon dioxide causes more than 60% of global warming 74. According to scientific reports, the concentration of CO2 is now about 400 ppm, which shows a significant increase from its concentration before industrialization, which was less than 300 ppm 74. Usually three solutions are offered to decrease total CO2 in the atmosphere; the first is using non-carbon energy resources (e.g. renewable energy and hydrogen). The second solution is to improve energy efficiency to reduce greenhouse gas emissions per unit energy consumption, which requires an efficient use of energy. The third is carbon capture and storage. This solution emphasizes the development of CO2 capture methods as well as sequestration technologies 75.

Capture of CO2 from a gas mixture can be carried out through a variety of methods, such as chemical and physical absorption, cryogenic fractionation and selective adsorption by solid adsorbents, membrane separation and hydration separation 76 (Fig11). Several technological problems, nevertheless, exist in dealing with possible large-scale execution of CO2 capture in power plants 77.

Natural gas usually includes a large quantity of heavy hydrocarbons such as methane, ethane, propane, isobutene, normal butane and a significant amount of CO2. Carbon dioxide must be taken out from this gas before it is used. This is because CO2 is very corrosive in the presence of water and quickly damages pipelines and equipment. It also decreases the waste pipeline capacity as well as the heating value of a natural gas stream 78.

There are different methods for CO2 capture depending on the conditions of the emitted gas. These methods include chemical absorption, physical adsorption, physical absorption, membrane separation as well as cryogenic separation. The chemical absorption method is one of the most frequently used commercial methods for CO2 capture and it is the most practical option in contrast to other methods for many industrial cases 79, 80. Most of these processes involve. the use of aqueous alkanolamine solutions. The CO2 absorption process usually consists of circulating the gases through a packed or tray column. Chemical absorption methods are the ones most used for CO2 removal from gases. These are very efficient and more economic in comparison with other methods 81.
Recently ionic liquids have been suggested as possible solvents for CO2 absorption due to their special attributes which are extensive liquid variety, thermal steadiness, tunable physicochemical disposition and high CO2 solubility 82. Brennecke’s group mentioned that CO2 was extremely soluble in 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), gaining a mole division of 0.6 at 8 MPa 83. However, the superior CO2 solubility in 1-n-butyl-3-methylimidazolium tetrafluoroborate (bmimBF4) compared to C2H6, CH4, O2, N2, H2 and CO, as mentioned by Jacquemin et al. 84, at near atmospheric pressures implies the superior discerning absorption.

In 2002, Davis Jr. and his co-workers proposed the concept of a functionalised ionic liquids 85 Initially, they incorporated an amine functionality onto the terminal end of the side chain of an imidazolium cation. Coupled with the tetrafluoroborate anion, this generated the functionalised ionic liquid shown in Scheme 12. This ionic liquid was shown to result in the chemisorption of carbon dioxide at a 1:2 CO2:IL ratio.